This project is being conducted to demonstrate the presence of methanogenic bacteria in the dental plaque of monkeys (Macaca fascicularis and Macaca mulatta) and human periodontal patients. The work completed to date using enrichment culture techniques has demonstrated the presence and metabolic activities of these methanogenic bacteria and yielded human, methanogen isolates. The work is being directed towards the taxonomic characterization of the pure cultures and the establishment of potential interactions between the oral methanogens and other strains of oral bacteria. The taxonomic experiments completed to date have established that the human isolates are Gram positive cocci in short chains. The isolates are metabolically and morphologically similar but immunologically distinct, (ABK1), from the genus Methanobrevibacter. Preliminary results have been collected with respect to the participation of the human isolates in hydrogen and carbon transfer reactions. Binary mixtures of isolates ABK1 or ABK4 and Veillonella rodentium exhibited methanogenesis upon addition of lactic acid. Tertiary mixtures of ABK1, V. rodentium and oral saccharolytic bacteria resulted in methanogenesis when glucose was added to the mixture. Teriary mixtures of ABK4, V. rodentium and saccharolytic strains revealed a utilization of ethanol as a methanogenic substrate.